In the projection of intergrated circuits it is common, as described in the aforementioned copending applications, to utilize a semiconductive wafer, e.g., a wafer cut from a zone-refined bar of silicon, to coat this wafer with a photosensitive layer, .e.g. a photoresist, and to expose this layer to a pattern which is reduced optically from a mask in a projection head.
Frequently, the pattern is projected onto successive regions of the wafer which is moved, more or less in the wafer plane, on a support stage provided with X and Y cross-slides for positioning each section in order along the optical axis.
Following exposure, the layer is developed and the developed layer constitutes a representation of the mask patterns and serves as a resist when the wafer is subjected to etching or further treatments, e.g. diffusion of materials into the wafer or transformation of the wafer structure in production of the semiconductor elements adapted to form the integrated circuit. The various chemical and physical treatments to which the resist-covered wafer may be exposed are well known in the art and require no elucidation here.
Since the mask which establishes the projection pattern which is to be generated in each region of the resist is itself generally an optical reduction from a master, it is apparent that high precision is required during the exposure or projection copying operation.
This high precision involves not only the optical or projection system and, as has been described in some of the aforementioned copending applications, the means for focusing the image onto the photoelectric layer or the means for positioning the wafer upon the support stage, but also the stage itself. In practice, the permissible tolerances in positioning the successive regions of the wafer along the optical axis are less than one micron and the photographic reduction in generating the image upon the wafer may be a factor of 10 as well.
In order to keep these tolerances and the requisite high precision, it has been found to be advantageous to project each image individually upon the respective region and hence to position each region in line with the optical axis after the image projection of a prior region generally contiguous to the region next exposed to the projection beam.
This is done, as alluded to earlier, by a stage capable of holding the wafer and having a cross-feed arrangement, i.e. a pair of carriages which are displaceable perpendicularly to one another so that one carriage can be displaced in a direction which, for convenience in defining coordinates, can be considered the X direction while the other carriage is displaceable in the Y direction, both mutually perpendicular directions being in the horizontal plane. Means is generally provided in the projection head of the apparatus to permit setting the focus at the wafer plane, i.e. for producing any necessary adjustment of the focal point in this plane in the vertical or Z direction.
The high precision can be obtained by controlling the X-Y cross-slide or stage using laser interferometric techniques.
However, even in such systems problems have been encountered with respect to the precise orientation of the upper surface of the wafer at the optical axis. It should immediately be apparent that high precision can only be obtained if the plane of this upper surface is normal to the optical axis.
In practice, however, the surface of the wafer may be irregular or irregularities may be formed in the layer of photosensitive materials applied to the wafer.
Thus, utilizing conventional devices, the plane of the region lined up with the optical axis frequently was not normal to this axis or truly perpendicular thereto and defective exposures resulted.
In European patent No. 27,570, therefore, a system has been proposed in which the chuck which retains the wafer, e.g. a vacuum chuck, was tiltable or pivotal about a fixed axis through the center of the wafer.
This represented a significant advance because at least at the center of the wafer, practically precise perpendicularity between the exposure plane and the optical axis could be ensured.
However, when large wafers were exposed along the periphery or at distances from the center of the wafer, difficulties were encountered, especially where intrinsic irregularities in the wafer surface were present.
Efforts to utilize the tiltability of the chuck to correct for lack of perpendicularity between the plane of the surface to be exposed on the optical axis resulted in shifting of the wafer or an incomplete compensation.
It will be appreciated that for maximum sharpness of the exposure, a global leveling of the surface by swinging the chuck about an axis at the center of the wafer is not sufficient.
With high resolving power optical systems (less than 1.5 microns) the depth of field is only about 2 microns. This depth of field can be taken up readily by irregularities in the substrate surface and in the thicknss of the photosensitive layer so that there is no depth of field reserved for compensating the bending or like irregularities of the wafer.
As a consequence, it is imperative that each image field for each exposure, i.e. each region of the wafer to be exposed, be individually leveled. This cannot be achieved utilizing a tilting system capable of tilting the chuck only about the center point of the wafer.
More specifically, when an effort is made to level each individual projection region of the wafer utilizing this single pivot point system of European Patent No. 27570, a tilt will result in an undesired shift in the Z (vertical direction), especially where this region is remote from the center of the wafer, requiring a renewed focusing action.
Still more disadvantageous is that the apparatus cannot adequately position the wafer plane so that it is perpendicular to the obstacle axis, nor can this be achieved rapidly utilizing the conventional device.
The system described in German open application No. 29 05 635 attempted to overcome the problem of leveling discrete regions of the wafer in the sense described above although this system is complex and practical realization of the means for carrying out the method is lacking in this disclosure.
Mention should also be made of the fact that it is known to level the wafer not only by movement of the chuck supporting the same, but also of the entire table or cross-slide arrangement (stage) utilized for shifting the wafer in the X and Y directions.
Such systems have generally required tilting about axes substantially offset from the wafer plane so that any leveling movement frequently resulted in undesired displacements of the wafer along other axes.